Regulators put the ceiling at 400 mg a day, about four cups of coffee
Two agencies reached 400 mg a day, and 200 mg in one sitting, separately, from a review of more than 300 human and animal studies. It is a population figure, not a personal one.
What the study found
400mg of caffeine a day, and 200mg at once, is the official line
Regulators put the ceiling for healthy adults at 400mg of caffeine a day, roughly four cups of coffee, and 200mg in any one sitting. The figure comes from a review of more than 300 human and animal studies, and the European and US agencies arrived at it separately.
The caveat · Clearance speed differs by person
This is a threshold for a population of healthy, non-pregnant adults rather than a personal limit; the same 400mg lands differently on people who clear caffeine at different speeds. It is regulatory guidance rather than the result of a single trial. The 200mg single-serving figure rests on thinner data than the daily one.